This invention is directed at laminate structures, and particularly at laminate structures suitable for use under retort conditions. The packaging industry has expended enormous sums of money over many years attempting to develop and commercialize retortable pouches.
A typical retort pouch is made of face-to-face laminate structures which are sealed together on three sides before filling, and are sealed on the fourth side after filling. The sealed retort pouches usually are heated or retorted after final sealing to cook the contents of the pouch and to kill all organisms within the pouch.
A significant problem encountered in development of retort pouches is the construction of the multiple layer laminate structure from which the pouch is formed. Typical laminate structures comprise an aluminum foil with a polyester adhesively attached to one side thereof, frequently by means of a polyester urethane adhesive, and a sealable lamina attached to the second side of the foil.
If all that was desired was the structuring of a pouch, the aluminum foil could be attached to the sealable lamina in the same manner as it is attached to the polyester lamina, namely by a polyester urethane adhesive. However, polyester urethanes have not been approved by governmental authorities because of possible contamination of the contained food. Therefore, for retort pouches for enclosing food, a primer of maleic anhydride polymer grafted onto polypropylene is conventionally coated on the second surface of the aluminum foil, and then it is heat-cured in line with the coating operation. Such materials are known by the trade names Hercorprime and Morprime. An inner sealant layer of polypropylene may then be attached onto the primer at high temperatures on the order of 500.degree. Fahrenheit. Unfortunately, the high temperature heating of the sealant layer of polypropylene appears to oxidize the surface of the polypropylene, thereby increasing the difficulty of sealing the laminate structures together to form the pouch.
In a recent development, a separate layer of polypropylene is extruded into a combining nip as an adhesive between a previously formed sealant layer of polypropylene and the cured primer. While this recent development appears to have solved the heat sealing problem, making the pouch functional, it is desirable to reduce the cost of the structure in order to enhance its competitive position relative to other packaging structures.
In the recent development cited above, constructing the laminate requires six steps.
(1) coat foil with primer liquid
(2) evaporate primer solvent
(3) cure primer in oven
(4) adhesive laminate unprimed side of foil to polyester
(5) extrude sealant layer
(6) extrusion laminate the final structure together.
It is an object of this invention to reduce the number of processing steps required in making a laminate suitable for use in retortable pouches.
Another object is to reduce the handling of unsupported metal foil both in machine operations, and in moving the foil between machines for subsequent operations and treatments in construction of the laminate.